I realized by now it can't be done at the protocol level. But it could
be done by the client library. Not as a "mandatory" option, but an
initial default. That would be sufficient for me.

Yes, a "TLS on/off" ldap.conf option. We'd also need an anti-"-Z"
command line option too to turn it off. Also it would be useful if the
-Z (and "TLS on") options were ignored when using 'ldaps:' URLs.